Aerosol, Sundarbans: Fine mode aerosol chemistry over a tropical urban atmosphere: characterization of ionic and carbonaceous species, A. Chatterjee & C. Dutta & T. K. Jana & S. Sen, J Atmos Chem DOI 10.1007/s10874-012-9231-8, 2012. An extensive aerosol sampling program was conducted during January-December 2006 over Kolkata (22º33′ N and 88º20′ E), a mega-city in eastern India in order to understand the sources, distributions and properties of atmospheric fine mode aerosol (PM2.5). The primary focus of this study is to determine the relative contribution of natural and anthropogenic as well as local and transported components to the total fine mode aerosol loading and their seasonal distributions over the metropolis. The average concentrations of fine mode aerosol was found to be 71.2±25.2 μgm-3 varying between 34.5 μgm-3 in monsoon and 112.6 μgm-3 in winter. The formation pathways of major secondary aerosol components like nitrate and sulphate in different seasons are discussed. A long range transport of dust aerosol from arid and semiarid regions of western India and beyond was observed during pre-monsoon which significantly enriched the total aerosol concentration. Vehicular emissions, biomass burning and transported dust particles were the major sources of PM2.5 from local and continental regions whereas seasalt aerosol was the major source of PM2.5 from marine source regions.